1. Field of the Invention
The present invention relates generally to a mobile communication system to which Automatic Repeat reQuest (ARQ) is applied, and in particular, to a method and apparatus for efficiently using a sequence number used for ARQ in packet transmission/reception.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that employs Wideband Code Division Multiple Access (WCDMA) and is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are European mobile communication systems.
In 3rd Generation Partnership Project (3GPP) in charge of UMTS standardization, Long Term Evolution (LTE), the next generation mobile communication system of the UMTS system, is now under discussion. LTE is a technology for implementing high-speed packet-based communication at a data rate of a maximum of 100 Mbps, aiming at deployment in around 2010. To this end, several plans are under discussion, including one plan to reduce the number of nodes located in a transmission path by simplifying a configuration of the network, and another plan to approach radio protocols as close to radio channels as possible.
FIG. 1 is a diagram illustrating an exemplary configuration of an Evolved UMTS mobile communication system to which the present invention is applicable.
Referring to FIG. 1, Evolved UMTS Radio Access Networks (E-RANs) 110 and 112 each are simplified to a 2-node configuration of Evolved Node Bs (ENBs or Node Bs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 by means of the E-RANs 110 and 112.
The ENBs 120 to 128 each correspond to the existing Node B of the UMTS system, and are connected to the UE 101 over a wireless channel. Compared to the existing Node B, the ENBs 120 to 128 perform complex functions. In LTE, all user traffic, including a real-time service such as Voice over IP (VoIP), is serviced over a shared channel, so there is a need for an apparatus for collecting status information of UEs and performing scheduling depending thereon. The scheduling is managed by the ENBs 120 to 128.
In order to implement a data rate of a maximum of 100 Mbps, the LTE system can use Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in the 20-MHz bandwidth. In addition, an Adaptive Modulation & Coding (AMC) scheme for determining a modulation scheme and a channel coding rate according to a channel status of the UE can be applied to LTE.
Like the mobile communication system supporting High-Speed Downlink Packet Access (HSDPA) or Enhanced uplink Dedicated CHannel (E-DCH) service, the LTE system also performs Hybrid ARQ (HARQ) between the ENBs 120 to 128 and the UE 101. However, because it is not possible to satisfy various Quality-of-Service (QoS) requirements only with HARQ, outer ARQ can be performed in an upper layer, and the outer ARQ can also be performed between the UE 101 and the ENBs 120 to 128.
As described above, many next generation mobile communication systems including the LTE system can use both HARQ and ARQ as an error correction technique.
HARQ refers to a technique of soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing a reception success rate. More specifically, an HARQ receiving side determines whether there is any error in a received packet, and then sends an HARQ positive ACKnowledgement (HARQ ACK) signal or an HARQ Negative ACKnowledgement (HARQ NACK) signal to a transmitting side according to presence/absence of the error. The transmitting side performs retransmission of the HARQ packet or transmission of a new HARQ packet according to the HARQ ACK/NACK signal. The HARQ receiving side soft-combines the retransmitted packet with the previously received packet, thereby reducing an error occurrence rate.
However, ARQ refers to a technique of checking a sequence number of a received packet, and requesting retransmission for the missing packet (or reception-failed packet), and this technique does not soft-combine the previously received packet with the retransmitted packets.
Particularly, in the LTE system, the ARQ operation is managed by a Radio Link Control (RLC) protocol layer, and the HARQ operation is managed by a Medium Access Control (MAC) or physical layer.
FIG. 2 is a diagram illustrating a protocol structure for an LTE system to which the present invention is applicable.
Referring to FIG. 2, the protocol structure can be established as a transmitting protocol structure (i.e., a protocol structure for a transmitting side) for a downlink service or a receiving protocol structure (i.e., a protocol structure for a receiving side) for an uplink service. In the following description, therefore, the protocol structures for the transmitting side and the receiving side will not be limited to any one of a UE (or terminal) and a Node B (or base station). In addition, both ‘entity’ and ‘layer’ are used as the terms indicating the same structure. Particularly, the term ‘entity’ stresses that a corresponding layer can be composed of a plurality of blocks, and the blocks operate independently of each other. However, the term ‘layer’, though it is identical to the term ‘entity’ in meaning, stresses the function of the corresponding layer rather than the independent operations of the blocks. Therefore, the ‘entity’ and the ‘layer’ are defined as the same terms, and used without distinction. In the LTE system, one Packet Data Convergence Protocol (PDCP) entity 205, 210, 215, 280, 285 and 290, and one Radio Link Control (RLC) entity 220, 225, 230, 265, 270 and 275 are configured per service.
The PDCP entities 205, 210, 215, 280, 285 and 290 take charge of an IP header compression/decompression operation, and the RLC entities 220, 225, 230, 265, 270 and 275 configure a PDCP Packet Data Unit (PDU) in an appropriate size, and perform an ARQ operation thereon. The PDU herein indicates a packet output from a particular protocol entity, and the PDCP PDU refers to a packet output from the PDCP entity.
MAC layers 235 and 260 are connected to several RLC entities 220, 225, 230, 265, 270 and 275 formed in one UE, and perform an operation of multiplexing multiple RLC PDUs output from the RLC entities 220, 225, 230, 265, 270 and 275 into a MAC PDU, or demultiplexing RLC PDUs from the MAC PDU.
HARQ layers 240 and 250 transmit/receive the MAC PDU through a predetermined HARQ operation. Physical layers 245 and 250 perform an operation of channel-coding/modulating upper layer data into an OFDM symbol and transmitting the OFDM symbol over a wireless channel, or demodulating/channel-decoding an OFDM symbol received over a wireless channel and delivering the channel-decoded OFDM symbol to an upper layer.
FIG. 3 is a diagram illustrating an RLC PDU used in an RLC layer.
Referring to FIG. 3, a basic unit of ARQ, used in an RLC layer, is called an RLC PDU 305.
The RLC layer makes a payload 320 by slicing or concatenating data such as PDCP PDU, provided from an upper layer, in an appropriate size, and makes the RLC PDU 305 by inserting a Sequence Number (SN) 310 and other necessary header 315 into the payload 320.
The RLC PDU 305 is transmitted to a receiving RLC entity (i.e., an RLC entity of a receiving side) through a MAC layer and a physical layer. The receiving RLC entity determines a success/failure in packet reception using the sequence number 310 of the RLC PDU 305, and then sends a report to a transmitting RLC entity (i.e., an RLC entity of a transmitting side), and the transmitting RLC entity retransmits an RLC PDU corresponding to a missing sequence number reported by the receiving RLC entity.
Therefore, the sequence number 310 used for the RLC PDU 305 should be unambiguously distinguished from sequence numbers of other RLC PDUs.
FIG. 4 is a diagram for a description of a wraparound phenomenon based on a method of allocating a sequence number.
Referring to FIG. 4, an RLC layer sets a sequence number of an RLC PDU using finite or predetermined bits. That is, the number of bits used for the sequence number of the RLC PDU is finite, and if the sequence number monotonously increases one by one and reaches a predetermined value, it returns back to ‘0’ (or makes a wraparound).
For example, if a sequence number has a k-bit size, the sequence number monotonously increases one by one from 0 (indicated by reference numeral 410) to (2k−1) (indicated by reference numeral 415). Thereafter, the sequence number returns to ‘0’ indicated by reference numeral 420, and monotonously increases again one by one.
Because the sequence number is repeatedly used as described above, the size of the sequence number should be large enough to avoid confusion due to the repeated use of the sequence number and to unambiguously distinguish an arbitrary RLC PDU.
Therefore, particularly in the next generation mobile communication system using ARQ, the sequence number should be large enough in size for the following reason. That is, if an arbitrary value is used as a sequence number of the RLC PDU at an arbitrary time, it is not possible to any longer retransmit an RLC PDU with a sequence number having the same value as that used in the previous period.
Generally, UMTS uses a 12-bit sequence number, and the conventional retransmission protocol such as Transmission Control Protocol (TCP) uses a 16-bit sequence number.
Although a size of the sequence number to be used in the LTE system has not been determined yet, there is a high possibility that a size similar to the size of the sequence number used in the foregoing conventional system will be used.
However, the sequence number with the 12 to 16-bit size may have no problem in a good channel condition, but may cause excessively high overhead in a poor channel condition. According to the current discussion on LTE, a packet with the smallest size, transmitted for one transmission period, is expected to have about 100 bits, and in this case, the overhead caused by the 12 to 16-bit sequence number exceeds 10%. Therefore, there is a need for a method for efficiently using a sequence number of a desired transmission packet in the next generation mobile communication system such as the LTE system.